Online food retailer Ocado has sourced 100,000 coronavirus tests for its workers from China, it has emerged, raising concerns over their quality after thousands of other kits bought from the country have had to be returned because they were substandard.

But the Guardian has established that the tests – which cost £15 each, much less than other kits – have been sourced from China, where tens of thousands have already been returned by the Spanish government over quality issues.

Ocado confirmed the tests came from China but declined to name which firm it had bought them from or why they appeared to be so cheap. About 60,000 of the tests have not yet been delivered. The union representing Ocado’s workers is now urgently seeking assurances over the quality of the tests.

Asked about where the testing kits had been sourced from and why they were so cheap, an Ocado Group spokesman confirmed to the Guardian: “They’re from suppliers in China.”

The spokesman was unable to account for the significantly cheaper price. The kits will be used to test Ocado delivery drivers and grocery packers, both of whom are classed as “key workers” under government guidelines, affording them certain privileges such as permission to send their children to school.

News of the Ocado kits came as concern grows over frontline NHS workers, including doctors and nurses, accessing coronavirus tests. Ocado has already promised to hand over its tests to the NHS if required.

A spokesman for the Union of Shop Distributive & Allied Workers (Usdaw), which represents Ocado workers, said: “While recognising that the company’s intention is to provide reassurance for staff, Usdaw is seeking urgent assurances from Ocado over the quality of their Covid-19 testing kits, as it is widely acknowledged that poor tests can be more problematic than no tests at all.

“Usdaw remains of the view that the best way to provide reassurance to staff is to ensure they can make decisions based on their health and wellbeing. To that end, the union welcomed the company’s decision to amend their policies to provide greater certainty to staff during this time, including pay from day one during self-isolation. The union continues to advise members to observe the necessary hygiene and social distancing advice from government, regardless of any test results.”